1. Field of the Invention
The present invention relates to an apparatus and method for Automatic Gain Control (AGC). More particularly, the present invention relates to an AGC apparatus and method in a Broadband Wireless Access (BWA) communication system.
2. Description of the Related Art
The provisioning of services with diverse Quality of Service (QoS) levels at or above 100 Mbps to users is an active study area for a future-generation communication system called a 4th Generation (4G) communication system. Particularly, active research is conducted on the provisioning of high-speed service by ensuring mobility and QoS to a BWA communication system such as Wireless Local Area Network (WLAN) and Wireless Metropolitan Area Network (WMAN). Some major examples are Institute of Electrical and Electronics Engineers (IEEE) 802.16d and IEEE 802.16e for Worldwide Interoperability Microwave Access (WiMAX) or Wireless Broadband (WiBro).
The IEEE 802.16d and IEEE 802.16e communication systems adopt Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) for physical channels of the WMAN system in order to support a broadband transmit network. IEEE 802.16d considers only a single-cell structure with no regard to mobility of Subscriber Stations (SSs). In contrast, IEEE 802.16e supports the SSs' mobility to the IEEE 802.16d communication system. A mobile SS is called an MS. Hereinafter, an SS and an MS are referred to as a user terminal.
Typically, a receiver in a wireless communication system includes a variable gain amplifier, particularly an automatic gain controller for automatically controlling the gain of a received signal. The AGC keeps the signal in the dynamic range of a device and provides the signal at a predetermined level to a signal detector.
FIG. 1 is a block diagram of a typical automatic gain controller.
Referring to FIG. 1, a detector 120 measures the energy of a signal received from a Gain Control Amplifier (GCA) 110 and calculates the difference between the energy measurement and a reference value −VR. The GCA 110 receives output data of HF(w) 100, the HF(w) 100 is a system function.
A filter 125 filters the difference so that the energy of the output signal of the GCA 110 equals the reference value −VR and provides the resulting signal as a control signal to the GCA 110.
FIG. 2 is a block diagram of a typical receiver.
Referring to FIG. 2, a Low Noise Amplifier (LNA) 202 and a first Band Pass Filter 203 (BPF 1) process a Radio Frequency (RF) signal received through an antenna 201. A frequency converter 204 downconverts the processed signal to an Intermediate Frequency (IF) signal.
A second BPF 205 (BPF 2) eliminates noise from the IF signal. A GCA 206 amplifies the noise-free signal and a frequency converter 207 downconverts the amplified signal to a baseband signal.
A Low Pass Filter (LPF) 208 processes the baseband signal, an Analog-to-Digital Converter (ADC) 209 samples the processed signal, and a demodulator 210 demodulates the samples.
FIG. 3 illustrates the basic structure of a downlink frame in a typical IEEE 802.16 communication system.
Referring to FIG. 3, an IEEE 802.16 frame largely includes a preamble symbol, data symbols carrying frame information, and other data symbols carrying user data. A beamformed symbol period can be detected from the frame based on a DownLink-MAP (DL-MAP) carrying frame configuration information in data symbols at the start of the frame.
FIG. 4 is a block diagram of a transmitter in a typical OFDM communication system.
Referring to FIG. 4, an encoder 410 channel-encodes input information bits at a predetermined coding rate in order to render the transmission data robust over a wireless channel. The channel encoding may involve interleaving aiming at robustness against burst errors.
A modulator 415 modulates the interleaved data in a modulation scheme such as Quadrature Phase Shift Keying (QPSK), 16-ary Quadrature Amplitude Modulation (16QAM), or 64-ary QAM (64QAM).
A subcarrier mapper 420 maps the modulated data to subcarriers. An Inverse Fast Fourier Transform (IFFT) processor 425 converts the mapped data to time sample data by IFFT.
A filter 430 is used to make the IFFT signals distinctive and stable. A Digital-to-Analog Converter (DAC) 435 converts the digital signal received from the filter 430 to an analog signal. An RF processor 440 upconverts the analog baseband signal to an RF signal transmittable in the air and transmits the RF signal through an antenna.
The transmission data is carried in an agreed format frame. In the frame, a preamble symbol is composed of pilot subcarriers and a data symbol is composed of data subcarriers and pilot subcarriers.
Concerning the data symbol, transmit power changes depending on transmission data allocation. With the time-domain gain control in the automatic gain controller illustrated in FIG. 1, however, a channel-incurred power change in a received signal cannot be compensated for successfully.
Accordingly, the OFDM communication system such as an IEEE 802.16 communication system performs automatic gain control on a frame basis using a preamble symbol in each frame that is transmitted with a constant transmit power. That is, the gain of an entire frame is controlled using the power measurement of a preamble symbol included in the frame.
FIG. 5 is a block diagram of a receiver in the typical OFDM communication system.
Referring to FIG. 5, an RF processor 510 downconverts an RF signal received through an antenna to a baseband signal. An ADC 515 converts the baseband analog signal to digital time sample data.
A filter 520 is used to make the digital signal distinctive and stable. A Fast Fourier Transform (FFT) processor 525 converts the time sample data received from the filter 520 to frequency data by FFT.
A subcarrier demapper 530 extracts data subcarriers carrying actual data from the frequency data. A demodulator 535 demodulates the data subcarriers in a predetermined demodulation scheme. A decoder 540 channel-decodes the demodulated data at a predetermined coding rate, thus recovering information data. The decoding may involve deinterleaving.
The IEEE 802.16 communication system usually uses beamforming in order to improve the reception performance of a user terminal. The beamforming applies only to predetermined symbols in a frame, effecting reception of a strong signal of 6 to 12 dB at the user terminal.
However, the frame-based AGC is not viable for a beamformed symbol period covering a predetermined number of symbols in a frame. Therefore, a large dynamic range must be designed for a received signal.
Accordingly, there exists a need for an improved apparatus and method for performing automatic gain control by measuring signal characteristics of a beamformed symbol period and controlling a dynamic range for a beamformed signal based on the measurement.